L'Hospital's Rule application with raised exponents.
up vote
4
down vote
favorite
I am having a little trouble going about:
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x}$$
Using $ln$ properties we can bring down the $10x$ exponent and have:$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
And from here I get stuck trying to apply L'Hospital's Rule to find the limit.
calculus limits derivatives
edited Dec 4 at 20:16
Bernard
117k637109
asked Dec 4 at 20:05
pijoborde
376
add a comment |
up vote
4
down vote
favorite
I am having a little trouble going about:
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x}$$
Using $ln$ properties we can bring down the $10x$ exponent and have:$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
And from here I get stuck trying to apply L'Hospital's Rule to find the limit.
calculus limits derivatives
edited Dec 4 at 20:16
Bernard
117k637109
asked Dec 4 at 20:05
pijoborde
376
You don't necessarly need l'Hopital here and I suggest to avoid that when we can proceed by standard limits.
– gimusi
Dec 4 at 20:35
add a comment |
up vote
4
down vote
favorite
up vote
4
down vote
favorite
I am having a little trouble going about:
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x}$$
Using $ln$ properties we can bring down the $10x$ exponent and have:$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
And from here I get stuck trying to apply L'Hospital's Rule to find the limit.
calculus limits derivatives
edited Dec 4 at 20:16
Bernard
117k637109
asked Dec 4 at 20:05
pijoborde
376
I am having a little trouble going about:
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x}$$
Using $ln$ properties we can bring down the $10x$ exponent and have:$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
And from here I get stuck trying to apply L'Hospital's Rule to find the limit.
calculus limits derivatives
calculus limits derivatives
edited Dec 4 at 20:16
Bernard
117k637109
asked Dec 4 at 20:05
pijoborde
376
edited Dec 4 at 20:16
Bernard
117k637109
asked Dec 4 at 20:05
pijoborde
376
edited Dec 4 at 20:16
Bernard
117k637109
edited Dec 4 at 20:16
Bernard
117k637109
edited Dec 4 at 20:16
Bernard
117k637109
117k637109
asked Dec 4 at 20:05
pijoborde
376
asked Dec 4 at 20:05
pijoborde
376
asked Dec 4 at 20:05
pijoborde
376
376
You don't necessarly need l'Hopital here and I suggest to avoid that when we can proceed by standard limits.
– gimusi
Dec 4 at 20:35
add a comment |
You don't necessarly need l'Hopital here and I suggest to avoid that when we can proceed by standard limits.
– gimusi
Dec 4 at 20:35
You don't necessarly need l'Hopital here and I suggest to avoid that when we can proceed by standard limits.
– gimusi
Dec 4 at 20:35
You don't necessarly need l'Hopital here and I suggest to avoid that when we can proceed by standard limits.
– gimusi
Dec 4 at 20:35
add a comment |
5 Answers
5
active
oldest
votes
up vote
4
down vote
accepted
Hint :
$$10x ln left(frac{14x}{14x+10}right)= frac{ln left(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
answered Dec 4 at 20:07
Rebellos
13.4k21142
add a comment |
up vote
3
down vote
HINT
We can use that
$$left(frac{14x}{14x+10}right)^{10x}=left(frac{14x+10-10}{14x+10}right)^{10x}=left[left(1-frac{10}{14x+10}right)^{frac{14x+10}{10}}right]^{frac{10cdot 10x}{14x+10}}$$
and refer to standard limits.
answered Dec 4 at 20:06
gimusi
91.7k84495
+1 by me since that was the original answer manipulating the definition of $e$.
– Rebellos
Dec 4 at 21:42
@Rebellos Thanks also your hint is very effective since the asker was looking for a method to apply l'Hopital rule! :)
– gimusi
Dec 4 at 21:45
I still don't get where e can come from here???
– pijoborde
Dec 4 at 21:56
@pijoborde Are you aware for as $nto infty$ $left(1+frac1nright)^n to e$ and $left(1+frac a nright)^n to e^a$?
– gimusi
Dec 4 at 21:58
And our a value is represented by 10 in this instance or? Making the answer e raised to 10? I mean, I feel like the answer is supposed to be e raised to some negative fraction.
– pijoborde
Dec 4 at 22:02
|
show 9 more comments
up vote
3
down vote
Hint: You want to reach some kind of indeterminate form, so you need to mess around with the fractions to reach a desirable form.
$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
$$ln y=frac{lnleft(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
Now, you can continue.
As an alternative approach (both faster and easier), you can use $lim_limits{n to infty}big(1+frac{x}{n}big)^n = e^x$. These expressions are easily manipulated to reach such a form.
Notice the numerator is $10$ less than the denominator, so $frac{14x}{14x+14} = 1+frac{-10}{14x+10}$. Hence, you get
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x} = lim_{xto infty} Biggl[left(1+frac{-10}{14x+10}right)^{14x+10}Biggl]^{frac{10x}{14x+10}}$$
answered Dec 4 at 20:26
KM101
3,609417
(+1) for the double hint
– gimusi
Dec 4 at 20:34
I am supposed to get an answer involving e. So I need to focus on your alternative approach method?
– pijoborde
Dec 4 at 20:45
@pijoborde You can use both method. I prefer and suggest the second one which is much more instructive.
– gimusi
Dec 4 at 20:46
I don't know if you've worked on the limit provided for $e^x$. If you're allowed to use it, why not? For all limits like this, just look for a way to reach something in the form $$lim_{n to infty}bigg[bigg(1+frac{x}{n}bigg)^nbigg]^m$$ and evaluate the limit from there. It's very simple and has little room for error.
– KM101
Dec 4 at 20:47
Your answers are complete, but in my honest opinion, I don't find much sense in answering the same thing as others have first done and just elaborating it. The first part of your answer was answered by me and the second part by Gimusi.
– Rebellos
Dec 4 at 21:42
|
show 2 more comments
up vote
2
down vote
Once you get to the logarithm, you need
$$
lim_{xtoinfty}frac{lndfrac{14x}{14x+10}}{dfrac{1}{10x}}
$$
which is in the form $0/0$ and l'Hôpital can be applied. However it's much simpler if the numerator is written as $ln14+ln x-ln(14x+10)$, in order to compute the derivative (you know the limit is $0$ anyhow). So we get
$$
lim_{xtoinfty}frac{dfrac{1}{x}-dfrac{14}{14x+10}}{-dfrac{1}{10x^2}}=
lim_{xtoinfty}-10x^2frac{14x+10-14x}{x(14x+10)}=lim_{xtoinfty}-frac{100x}{14x+10}
$$
When you have this limit, let me call it $l$, the one you started with is $e^l$.
answered Dec 4 at 21:27
egreg
176k1384198
What happened exactly in the last step? I don't quite get where 100 came from? Unless you are reducing an x from both top and bottom to 1 and then distributing the -10x to the 10, since the positive and minus 14x will cancel out?
– pijoborde
Dec 6 at 7:01
@pijoborde $10cdot10=100$
– egreg
Dec 6 at 9:01
add a comment |
up vote
1
down vote
Tips:
With equivalents, it's very fast:$$lnbiggl(frac{14x}{14x+10}biggr)=lnbiggl(1-frac{10}{14x+10}biggr)sim_infty -frac{10}{14x+10}sim_infty -frac{10}{14x}=-frac 57,$$
so, as equivalence is compatible with multiplication/division,
$$10xlnbiggl(frac{14x}{14x+10}biggr)sim_infty -frac{50x}{7x}to -frac{50}7.$$
Warning: equivalence is not compatible with addition/subtraction. It is compatible, under mild hypotheses, with composition on the left by logarithm.
answered Dec 4 at 20:25
Bernard
117k637109
1
Equivalents are fast but also dangerous to handle for limits.
– gimusi
Dec 4 at 20:33
Why? If one knows the rules… When I was a student, our professor used to say that L'Hospital's rule can be dangerous (there are some hypotheses no one ever thinks to check), and when it works, Taylor at order $1$ works as well.
– Bernard
Dec 4 at 20:39
1
I'm not referring of course to the use by an expert user like you but for not expert user it can be dangerous. Let consider for example the limit $frac{t-log(1+t)}{t^2}$ accorging to $log (1+t) sim t$ one could conclude that $$frac{t-log(1+t)}{t^2}sim frac{t-t}{t^2}=0$$ which is of course wrong.
– gimusi
Dec 4 at 20:44
1
I think you should always indicates that issue when you suggest to use equivalents to not expert users.
– gimusi
Dec 4 at 20:45
2
I'm not claiming that the method is wrong, I'm claiming that we need to be aware about it to handle them and a not expert user can easily make a mistake. I would suggest for example use them in the form $log(1+t)=t+o(t)$ in order to take into account the rermainder term. But it is only a suggestion of course.
– gimusi
Dec 4 at 20:55
|
show 1 more comment
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5 Answers
5
active
oldest
votes
5 Answers
5
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
4
down vote
accepted
Hint :
$$10x ln left(frac{14x}{14x+10}right)= frac{ln left(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
answered Dec 4 at 20:07
Rebellos
13.4k21142
add a comment |
up vote
4
down vote
accepted
Hint :
$$10x ln left(frac{14x}{14x+10}right)= frac{ln left(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
answered Dec 4 at 20:07
Rebellos
13.4k21142
add a comment |
up vote
4
down vote
accepted
up vote
4
down vote
accepted
Hint :
$$10x ln left(frac{14x}{14x+10}right)= frac{ln left(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
answered Dec 4 at 20:07
Rebellos
13.4k21142
Hint :
$$10x ln left(frac{14x}{14x+10}right)= frac{ln left(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
answered Dec 4 at 20:07
Rebellos
13.4k21142
answered Dec 4 at 20:07
Rebellos
13.4k21142
answered Dec 4 at 20:07
Rebellos
13.4k21142
answered Dec 4 at 20:07
Rebellos
13.4k21142
13.4k21142
add a comment |
add a comment |
up vote
3
down vote
HINT
We can use that
$$left(frac{14x}{14x+10}right)^{10x}=left(frac{14x+10-10}{14x+10}right)^{10x}=left[left(1-frac{10}{14x+10}right)^{frac{14x+10}{10}}right]^{frac{10cdot 10x}{14x+10}}$$
and refer to standard limits.
answered Dec 4 at 20:06
gimusi
91.7k84495
+1 by me since that was the original answer manipulating the definition of $e$.
– Rebellos
Dec 4 at 21:42
@Rebellos Thanks also your hint is very effective since the asker was looking for a method to apply l'Hopital rule! :)
– gimusi
Dec 4 at 21:45
I still don't get where e can come from here???
– pijoborde
Dec 4 at 21:56
@pijoborde Are you aware for as $nto infty$ $left(1+frac1nright)^n to e$ and $left(1+frac a nright)^n to e^a$?
– gimusi
Dec 4 at 21:58
And our a value is represented by 10 in this instance or? Making the answer e raised to 10? I mean, I feel like the answer is supposed to be e raised to some negative fraction.
– pijoborde
Dec 4 at 22:02
|
show 9 more comments
up vote
3
down vote
HINT
We can use that
$$left(frac{14x}{14x+10}right)^{10x}=left(frac{14x+10-10}{14x+10}right)^{10x}=left[left(1-frac{10}{14x+10}right)^{frac{14x+10}{10}}right]^{frac{10cdot 10x}{14x+10}}$$
and refer to standard limits.
answered Dec 4 at 20:06
gimusi
91.7k84495
+1 by me since that was the original answer manipulating the definition of $e$.
– Rebellos
Dec 4 at 21:42
@Rebellos Thanks also your hint is very effective since the asker was looking for a method to apply l'Hopital rule! :)
– gimusi
Dec 4 at 21:45
I still don't get where e can come from here???
– pijoborde
Dec 4 at 21:56
@pijoborde Are you aware for as $nto infty$ $left(1+frac1nright)^n to e$ and $left(1+frac a nright)^n to e^a$?
– gimusi
Dec 4 at 21:58
And our a value is represented by 10 in this instance or? Making the answer e raised to 10? I mean, I feel like the answer is supposed to be e raised to some negative fraction.
– pijoborde
Dec 4 at 22:02
|
show 9 more comments
up vote
3
down vote
up vote
3
down vote
HINT
We can use that
$$left(frac{14x}{14x+10}right)^{10x}=left(frac{14x+10-10}{14x+10}right)^{10x}=left[left(1-frac{10}{14x+10}right)^{frac{14x+10}{10}}right]^{frac{10cdot 10x}{14x+10}}$$
and refer to standard limits.
answered Dec 4 at 20:06
gimusi
91.7k84495
HINT
We can use that
$$left(frac{14x}{14x+10}right)^{10x}=left(frac{14x+10-10}{14x+10}right)^{10x}=left[left(1-frac{10}{14x+10}right)^{frac{14x+10}{10}}right]^{frac{10cdot 10x}{14x+10}}$$
and refer to standard limits.
answered Dec 4 at 20:06
gimusi
91.7k84495
answered Dec 4 at 20:06
gimusi
91.7k84495
answered Dec 4 at 20:06
gimusi
91.7k84495
answered Dec 4 at 20:06
gimusi
91.7k84495
91.7k84495
+1 by me since that was the original answer manipulating the definition of $e$.
– Rebellos
Dec 4 at 21:42
@Rebellos Thanks also your hint is very effective since the asker was looking for a method to apply l'Hopital rule! :)
– gimusi
Dec 4 at 21:45
I still don't get where e can come from here???
– pijoborde
Dec 4 at 21:56
@pijoborde Are you aware for as $nto infty$ $left(1+frac1nright)^n to e$ and $left(1+frac a nright)^n to e^a$?
– gimusi
Dec 4 at 21:58
And our a value is represented by 10 in this instance or? Making the answer e raised to 10? I mean, I feel like the answer is supposed to be e raised to some negative fraction.
– pijoborde
Dec 4 at 22:02
|
show 9 more comments
+1 by me since that was the original answer manipulating the definition of $e$.
– Rebellos
Dec 4 at 21:42
@Rebellos Thanks also your hint is very effective since the asker was looking for a method to apply l'Hopital rule! :)
– gimusi
Dec 4 at 21:45
I still don't get where e can come from here???
– pijoborde
Dec 4 at 21:56
@pijoborde Are you aware for as $nto infty$ $left(1+frac1nright)^n to e$ and $left(1+frac a nright)^n to e^a$?
– gimusi
Dec 4 at 21:58
And our a value is represented by 10 in this instance or? Making the answer e raised to 10? I mean, I feel like the answer is supposed to be e raised to some negative fraction.
– pijoborde
Dec 4 at 22:02
+1 by me since that was the original answer manipulating the definition of $e$.
– Rebellos
Dec 4 at 21:42
+1 by me since that was the original answer manipulating the definition of $e$.
– Rebellos
Dec 4 at 21:42
@Rebellos Thanks also your hint is very effective since the asker was looking for a method to apply l'Hopital rule! :)
– gimusi
Dec 4 at 21:45
@Rebellos Thanks also your hint is very effective since the asker was looking for a method to apply l'Hopital rule! :)
– gimusi
Dec 4 at 21:45
I still don't get where e can come from here???
– pijoborde
Dec 4 at 21:56
I still don't get where e can come from here???
– pijoborde
Dec 4 at 21:56
@pijoborde Are you aware for as $nto infty$ $left(1+frac1nright)^n to e$ and $left(1+frac a nright)^n to e^a$?
– gimusi
Dec 4 at 21:58
@pijoborde Are you aware for as $nto infty$ $left(1+frac1nright)^n to e$ and $left(1+frac a nright)^n to e^a$?
– gimusi
Dec 4 at 21:58
And our a value is represented by 10 in this instance or? Making the answer e raised to 10? I mean, I feel like the answer is supposed to be e raised to some negative fraction.
– pijoborde
Dec 4 at 22:02
And our a value is represented by 10 in this instance or? Making the answer e raised to 10? I mean, I feel like the answer is supposed to be e raised to some negative fraction.
– pijoborde
Dec 4 at 22:02
|
show 9 more comments
up vote
3
down vote
Hint: You want to reach some kind of indeterminate form, so you need to mess around with the fractions to reach a desirable form.
$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
$$ln y=frac{lnleft(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
Now, you can continue.
As an alternative approach (both faster and easier), you can use $lim_limits{n to infty}big(1+frac{x}{n}big)^n = e^x$. These expressions are easily manipulated to reach such a form.
Notice the numerator is $10$ less than the denominator, so $frac{14x}{14x+14} = 1+frac{-10}{14x+10}$. Hence, you get
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x} = lim_{xto infty} Biggl[left(1+frac{-10}{14x+10}right)^{14x+10}Biggl]^{frac{10x}{14x+10}}$$
answered Dec 4 at 20:26
KM101
3,609417
(+1) for the double hint
– gimusi
Dec 4 at 20:34
I am supposed to get an answer involving e. So I need to focus on your alternative approach method?
– pijoborde
Dec 4 at 20:45
@pijoborde You can use both method. I prefer and suggest the second one which is much more instructive.
– gimusi
Dec 4 at 20:46
I don't know if you've worked on the limit provided for $e^x$. If you're allowed to use it, why not? For all limits like this, just look for a way to reach something in the form $$lim_{n to infty}bigg[bigg(1+frac{x}{n}bigg)^nbigg]^m$$ and evaluate the limit from there. It's very simple and has little room for error.
– KM101
Dec 4 at 20:47
Your answers are complete, but in my honest opinion, I don't find much sense in answering the same thing as others have first done and just elaborating it. The first part of your answer was answered by me and the second part by Gimusi.
– Rebellos
Dec 4 at 21:42
|
show 2 more comments
up vote
3
down vote
Hint: You want to reach some kind of indeterminate form, so you need to mess around with the fractions to reach a desirable form.
$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
$$ln y=frac{lnleft(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
Now, you can continue.
As an alternative approach (both faster and easier), you can use $lim_limits{n to infty}big(1+frac{x}{n}big)^n = e^x$. These expressions are easily manipulated to reach such a form.
Notice the numerator is $10$ less than the denominator, so $frac{14x}{14x+14} = 1+frac{-10}{14x+10}$. Hence, you get
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x} = lim_{xto infty} Biggl[left(1+frac{-10}{14x+10}right)^{14x+10}Biggl]^{frac{10x}{14x+10}}$$
answered Dec 4 at 20:26
KM101
3,609417
(+1) for the double hint
– gimusi
Dec 4 at 20:34
I am supposed to get an answer involving e. So I need to focus on your alternative approach method?
– pijoborde
Dec 4 at 20:45
@pijoborde You can use both method. I prefer and suggest the second one which is much more instructive.
– gimusi
Dec 4 at 20:46
I don't know if you've worked on the limit provided for $e^x$. If you're allowed to use it, why not? For all limits like this, just look for a way to reach something in the form $$lim_{n to infty}bigg[bigg(1+frac{x}{n}bigg)^nbigg]^m$$ and evaluate the limit from there. It's very simple and has little room for error.
– KM101
Dec 4 at 20:47
Your answers are complete, but in my honest opinion, I don't find much sense in answering the same thing as others have first done and just elaborating it. The first part of your answer was answered by me and the second part by Gimusi.
– Rebellos
Dec 4 at 21:42
|
show 2 more comments
up vote
3
down vote
up vote
3
down vote
Hint: You want to reach some kind of indeterminate form, so you need to mess around with the fractions to reach a desirable form.
$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
$$ln y=frac{lnleft(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
Now, you can continue.
As an alternative approach (both faster and easier), you can use $lim_limits{n to infty}big(1+frac{x}{n}big)^n = e^x$. These expressions are easily manipulated to reach such a form.
Notice the numerator is $10$ less than the denominator, so $frac{14x}{14x+14} = 1+frac{-10}{14x+10}$. Hence, you get
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x} = lim_{xto infty} Biggl[left(1+frac{-10}{14x+10}right)^{14x+10}Biggl]^{frac{10x}{14x+10}}$$
answered Dec 4 at 20:26
KM101
3,609417
Hint: You want to reach some kind of indeterminate form, so you need to mess around with the fractions to reach a desirable form.
$$ln y=10xlnleft(frac{14x}{14x+10}right)$$
$$ln y=frac{lnleft(frac{14x}{14x+10}right)}{frac{1}{10x}}$$
Now, you can continue.
As an alternative approach (both faster and easier), you can use $lim_limits{n to infty}big(1+frac{x}{n}big)^n = e^x$. These expressions are easily manipulated to reach such a form.
Notice the numerator is $10$ less than the denominator, so $frac{14x}{14x+14} = 1+frac{-10}{14x+10}$. Hence, you get
$$lim_{xto infty} left(frac{14x}{14x+10}right)^{10x} = lim_{xto infty} Biggl[left(1+frac{-10}{14x+10}right)^{14x+10}Biggl]^{frac{10x}{14x+10}}$$
answered Dec 4 at 20:26
KM101
3,609417
edited Dec 4 at 20:31
answered Dec 4 at 20:26
KM101
3,609417
answered Dec 4 at 20:26
KM101
3,609417
answered Dec 4 at 20:26
KM101
3,609417
3,609417
(+1) for the double hint
– gimusi
Dec 4 at 20:34
I am supposed to get an answer involving e. So I need to focus on your alternative approach method?
– pijoborde
Dec 4 at 20:45
@pijoborde You can use both method. I prefer and suggest the second one which is much more instructive.
– gimusi
Dec 4 at 20:46
I don't know if you've worked on the limit provided for $e^x$. If you're allowed to use it, why not? For all limits like this, just look for a way to reach something in the form $$lim_{n to infty}bigg[bigg(1+frac{x}{n}bigg)^nbigg]^m$$ and evaluate the limit from there. It's very simple and has little room for error.
– KM101
Dec 4 at 20:47
Your answers are complete, but in my honest opinion, I don't find much sense in answering the same thing as others have first done and just elaborating it. The first part of your answer was answered by me and the second part by Gimusi.
– Rebellos
Dec 4 at 21:42
|
show 2 more comments
(+1) for the double hint
– gimusi
Dec 4 at 20:34
I am supposed to get an answer involving e. So I need to focus on your alternative approach method?
– pijoborde
Dec 4 at 20:45
@pijoborde You can use both method. I prefer and suggest the second one which is much more instructive.
– gimusi
Dec 4 at 20:46
I don't know if you've worked on the limit provided for $e^x$. If you're allowed to use it, why not? For all limits like this, just look for a way to reach something in the form $$lim_{n to infty}bigg[bigg(1+frac{x}{n}bigg)^nbigg]^m$$ and evaluate the limit from there. It's very simple and has little room for error.
– KM101
Dec 4 at 20:47
Your answers are complete, but in my honest opinion, I don't find much sense in answering the same thing as others have first done and just elaborating it. The first part of your answer was answered by me and the second part by Gimusi.
– Rebellos
Dec 4 at 21:42
(+1) for the double hint
– gimusi
Dec 4 at 20:34
(+1) for the double hint
– gimusi
Dec 4 at 20:34
I am supposed to get an answer involving e. So I need to focus on your alternative approach method?
– pijoborde
Dec 4 at 20:45
I am supposed to get an answer involving e. So I need to focus on your alternative approach method?
– pijoborde
Dec 4 at 20:45
@pijoborde You can use both method. I prefer and suggest the second one which is much more instructive.
– gimusi
Dec 4 at 20:46
@pijoborde You can use both method. I prefer and suggest the second one which is much more instructive.
– gimusi
Dec 4 at 20:46
I don't know if you've worked on the limit provided for $e^x$. If you're allowed to use it, why not? For all limits like this, just look for a way to reach something in the form $$lim_{n to infty}bigg[bigg(1+frac{x}{n}bigg)^nbigg]^m$$ and evaluate the limit from there. It's very simple and has little room for error.
– KM101
Dec 4 at 20:47
I don't know if you've worked on the limit provided for $e^x$. If you're allowed to use it, why not? For all limits like this, just look for a way to reach something in the form $$lim_{n to infty}bigg[bigg(1+frac{x}{n}bigg)^nbigg]^m$$ and evaluate the limit from there. It's very simple and has little room for error.
– KM101
Dec 4 at 20:47
Your answers are complete, but in my honest opinion, I don't find much sense in answering the same thing as others have first done and just elaborating it. The first part of your answer was answered by me and the second part by Gimusi.
– Rebellos
Dec 4 at 21:42
Your answers are complete, but in my honest opinion, I don't find much sense in answering the same thing as others have first done and just elaborating it. The first part of your answer was answered by me and the second part by Gimusi.
– Rebellos
Dec 4 at 21:42
|
show 2 more comments
up vote
2
down vote
Once you get to the logarithm, you need
$$
lim_{xtoinfty}frac{lndfrac{14x}{14x+10}}{dfrac{1}{10x}}
$$
which is in the form $0/0$ and l'Hôpital can be applied. However it's much simpler if the numerator is written as $ln14+ln x-ln(14x+10)$, in order to compute the derivative (you know the limit is $0$ anyhow). So we get
$$
lim_{xtoinfty}frac{dfrac{1}{x}-dfrac{14}{14x+10}}{-dfrac{1}{10x^2}}=
lim_{xtoinfty}-10x^2frac{14x+10-14x}{x(14x+10)}=lim_{xtoinfty}-frac{100x}{14x+10}
$$
When you have this limit, let me call it $l$, the one you started with is $e^l$.
answered Dec 4 at 21:27
egreg
176k1384198
What happened exactly in the last step? I don't quite get where 100 came from? Unless you are reducing an x from both top and bottom to 1 and then distributing the -10x to the 10, since the positive and minus 14x will cancel out?
– pijoborde
Dec 6 at 7:01
@pijoborde $10cdot10=100$
– egreg
Dec 6 at 9:01
add a comment |
up vote
2
down vote
Once you get to the logarithm, you need
$$
lim_{xtoinfty}frac{lndfrac{14x}{14x+10}}{dfrac{1}{10x}}
$$
which is in the form $0/0$ and l'Hôpital can be applied. However it's much simpler if the numerator is written as $ln14+ln x-ln(14x+10)$, in order to compute the derivative (you know the limit is $0$ anyhow). So we get
$$
lim_{xtoinfty}frac{dfrac{1}{x}-dfrac{14}{14x+10}}{-dfrac{1}{10x^2}}=
lim_{xtoinfty}-10x^2frac{14x+10-14x}{x(14x+10)}=lim_{xtoinfty}-frac{100x}{14x+10}
$$
When you have this limit, let me call it $l$, the one you started with is $e^l$.
answered Dec 4 at 21:27
egreg
176k1384198
What happened exactly in the last step? I don't quite get where 100 came from? Unless you are reducing an x from both top and bottom to 1 and then distributing the -10x to the 10, since the positive and minus 14x will cancel out?
– pijoborde
Dec 6 at 7:01
@pijoborde $10cdot10=100$
– egreg
Dec 6 at 9:01
add a comment |
up vote
2
down vote
up vote
2
down vote
Once you get to the logarithm, you need
$$
lim_{xtoinfty}frac{lndfrac{14x}{14x+10}}{dfrac{1}{10x}}
$$
which is in the form $0/0$ and l'Hôpital can be applied. However it's much simpler if the numerator is written as $ln14+ln x-ln(14x+10)$, in order to compute the derivative (you know the limit is $0$ anyhow). So we get
$$
lim_{xtoinfty}frac{dfrac{1}{x}-dfrac{14}{14x+10}}{-dfrac{1}{10x^2}}=
lim_{xtoinfty}-10x^2frac{14x+10-14x}{x(14x+10)}=lim_{xtoinfty}-frac{100x}{14x+10}
$$
When you have this limit, let me call it $l$, the one you started with is $e^l$.
answered Dec 4 at 21:27
egreg
176k1384198
Once you get to the logarithm, you need
$$
lim_{xtoinfty}frac{lndfrac{14x}{14x+10}}{dfrac{1}{10x}}
$$
which is in the form $0/0$ and l'Hôpital can be applied. However it's much simpler if the numerator is written as $ln14+ln x-ln(14x+10)$, in order to compute the derivative (you know the limit is $0$ anyhow). So we get
$$
lim_{xtoinfty}frac{dfrac{1}{x}-dfrac{14}{14x+10}}{-dfrac{1}{10x^2}}=
lim_{xtoinfty}-10x^2frac{14x+10-14x}{x(14x+10)}=lim_{xtoinfty}-frac{100x}{14x+10}
$$
When you have this limit, let me call it $l$, the one you started with is $e^l$.
answered Dec 4 at 21:27
egreg
176k1384198
answered Dec 4 at 21:27
egreg
176k1384198
answered Dec 4 at 21:27
egreg
176k1384198
answered Dec 4 at 21:27
egreg
176k1384198
176k1384198
What happened exactly in the last step? I don't quite get where 100 came from? Unless you are reducing an x from both top and bottom to 1 and then distributing the -10x to the 10, since the positive and minus 14x will cancel out?
– pijoborde
Dec 6 at 7:01
@pijoborde $10cdot10=100$
– egreg
Dec 6 at 9:01
add a comment |
What happened exactly in the last step? I don't quite get where 100 came from? Unless you are reducing an x from both top and bottom to 1 and then distributing the -10x to the 10, since the positive and minus 14x will cancel out?
– pijoborde
Dec 6 at 7:01
@pijoborde $10cdot10=100$
– egreg
Dec 6 at 9:01
What happened exactly in the last step? I don't quite get where 100 came from? Unless you are reducing an x from both top and bottom to 1 and then distributing the -10x to the 10, since the positive and minus 14x will cancel out?
– pijoborde
Dec 6 at 7:01
What happened exactly in the last step? I don't quite get where 100 came from? Unless you are reducing an x from both top and bottom to 1 and then distributing the -10x to the 10, since the positive and minus 14x will cancel out?
– pijoborde
Dec 6 at 7:01
@pijoborde $10cdot10=100$
– egreg
Dec 6 at 9:01
@pijoborde $10cdot10=100$
– egreg
Dec 6 at 9:01
add a comment |
up vote
1
down vote
Tips:
With equivalents, it's very fast:$$lnbiggl(frac{14x}{14x+10}biggr)=lnbiggl(1-frac{10}{14x+10}biggr)sim_infty -frac{10}{14x+10}sim_infty -frac{10}{14x}=-frac 57,$$
so, as equivalence is compatible with multiplication/division,
$$10xlnbiggl(frac{14x}{14x+10}biggr)sim_infty -frac{50x}{7x}to -frac{50}7.$$
Warning: equivalence is not compatible with addition/subtraction. It is compatible, under mild hypotheses, with composition on the left by logarithm.
answered Dec 4 at 20:25
Bernard
117k637109
1
Equivalents are fast but also dangerous to handle for limits.
– gimusi
Dec 4 at 20:33
Why? If one knows the rules… When I was a student, our professor used to say that L'Hospital's rule can be dangerous (there are some hypotheses no one ever thinks to check), and when it works, Taylor at order $1$ works as well.
– Bernard
Dec 4 at 20:39
1
I'm not referring of course to the use by an expert user like you but for not expert user it can be dangerous. Let consider for example the limit $frac{t-log(1+t)}{t^2}$ accorging to $log (1+t) sim t$ one could conclude that $$frac{t-log(1+t)}{t^2}sim frac{t-t}{t^2}=0$$ which is of course wrong.
– gimusi
Dec 4 at 20:44
1
I think you should always indicates that issue when you suggest to use equivalents to not expert users.
– gimusi
Dec 4 at 20:45
2
I'm not claiming that the method is wrong, I'm claiming that we need to be aware about it to handle them and a not expert user can easily make a mistake. I would suggest for example use them in the form $log(1+t)=t+o(t)$ in order to take into account the rermainder term. But it is only a suggestion of course.
– gimusi
Dec 4 at 20:55
|
show 1 more comment
up vote
1
down vote
Tips:
With equivalents, it's very fast:$$lnbiggl(frac{14x}{14x+10}biggr)=lnbiggl(1-frac{10}{14x+10}biggr)sim_infty -frac{10}{14x+10}sim_infty -frac{10}{14x}=-frac 57,$$
so, as equivalence is compatible with multiplication/division,
$$10xlnbiggl(frac{14x}{14x+10}biggr)sim_infty -frac{50x}{7x}to -frac{50}7.$$
Warning: equivalence is not compatible with addition/subtraction. It is compatible, under mild hypotheses, with composition on the left by logarithm.
answered Dec 4 at 20:25
Bernard
117k637109
1
Equivalents are fast but also dangerous to handle for limits.
– gimusi
Dec 4 at 20:33
Why? If one knows the rules… When I was a student, our professor used to say that L'Hospital's rule can be dangerous (there are some hypotheses no one ever thinks to check), and when it works, Taylor at order $1$ works as well.
– Bernard
Dec 4 at 20:39
1
I'm not referring of course to the use by an expert user like you but for not expert user it can be dangerous. Let consider for example the limit $frac{t-log(1+t)}{t^2}$ accorging to $log (1+t) sim t$ one could conclude that $$frac{t-log(1+t)}{t^2}sim frac{t-t}{t^2}=0$$ which is of course wrong.
– gimusi
Dec 4 at 20:44
1
I think you should always indicates that issue when you suggest to use equivalents to not expert users.
– gimusi
Dec 4 at 20:45
2
I'm not claiming that the method is wrong, I'm claiming that we need to be aware about it to handle them and a not expert user can easily make a mistake. I would suggest for example use them in the form $log(1+t)=t+o(t)$ in order to take into account the rermainder term. But it is only a suggestion of course.
– gimusi
Dec 4 at 20:55
|
show 1 more comment
up vote
1
down vote
up vote
1
down vote
Tips:
With equivalents, it's very fast:$$lnbiggl(frac{14x}{14x+10}biggr)=lnbiggl(1-frac{10}{14x+10}biggr)sim_infty -frac{10}{14x+10}sim_infty -frac{10}{14x}=-frac 57,$$
so, as equivalence is compatible with multiplication/division,
$$10xlnbiggl(frac{14x}{14x+10}biggr)sim_infty -frac{50x}{7x}to -frac{50}7.$$
Warning: equivalence is not compatible with addition/subtraction. It is compatible, under mild hypotheses, with composition on the left by logarithm.
answered Dec 4 at 20:25
Bernard
117k637109
Tips:
With equivalents, it's very fast:$$lnbiggl(frac{14x}{14x+10}biggr)=lnbiggl(1-frac{10}{14x+10}biggr)sim_infty -frac{10}{14x+10}sim_infty -frac{10}{14x}=-frac 57,$$
so, as equivalence is compatible with multiplication/division,
$$10xlnbiggl(frac{14x}{14x+10}biggr)sim_infty -frac{50x}{7x}to -frac{50}7.$$
Warning: equivalence is not compatible with addition/subtraction. It is compatible, under mild hypotheses, with composition on the left by logarithm.
answered Dec 4 at 20:25
Bernard
117k637109
edited Dec 5 at 18:43
answered Dec 4 at 20:25
Bernard
117k637109
answered Dec 4 at 20:25
Bernard
117k637109
answered Dec 4 at 20:25
Bernard
117k637109
117k637109
1
Equivalents are fast but also dangerous to handle for limits.
– gimusi
Dec 4 at 20:33
Why? If one knows the rules… When I was a student, our professor used to say that L'Hospital's rule can be dangerous (there are some hypotheses no one ever thinks to check), and when it works, Taylor at order $1$ works as well.
– Bernard
Dec 4 at 20:39
1
I'm not referring of course to the use by an expert user like you but for not expert user it can be dangerous. Let consider for example the limit $frac{t-log(1+t)}{t^2}$ accorging to $log (1+t) sim t$ one could conclude that $$frac{t-log(1+t)}{t^2}sim frac{t-t}{t^2}=0$$ which is of course wrong.
– gimusi
Dec 4 at 20:44
1
I think you should always indicates that issue when you suggest to use equivalents to not expert users.
– gimusi
Dec 4 at 20:45
2
I'm not claiming that the method is wrong, I'm claiming that we need to be aware about it to handle them and a not expert user can easily make a mistake. I would suggest for example use them in the form $log(1+t)=t+o(t)$ in order to take into account the rermainder term. But it is only a suggestion of course.
– gimusi
Dec 4 at 20:55
|
show 1 more comment
1
Equivalents are fast but also dangerous to handle for limits.
– gimusi
Dec 4 at 20:33
Why? If one knows the rules… When I was a student, our professor used to say that L'Hospital's rule can be dangerous (there are some hypotheses no one ever thinks to check), and when it works, Taylor at order $1$ works as well.
– Bernard
Dec 4 at 20:39
1
I'm not referring of course to the use by an expert user like you but for not expert user it can be dangerous. Let consider for example the limit $frac{t-log(1+t)}{t^2}$ accorging to $log (1+t) sim t$ one could conclude that $$frac{t-log(1+t)}{t^2}sim frac{t-t}{t^2}=0$$ which is of course wrong.
– gimusi
Dec 4 at 20:44
1
I think you should always indicates that issue when you suggest to use equivalents to not expert users.
– gimusi
Dec 4 at 20:45
2
I'm not claiming that the method is wrong, I'm claiming that we need to be aware about it to handle them and a not expert user can easily make a mistake. I would suggest for example use them in the form $log(1+t)=t+o(t)$ in order to take into account the rermainder term. But it is only a suggestion of course.
– gimusi
Dec 4 at 20:55
1
1
Equivalents are fast but also dangerous to handle for limits.
– gimusi
Dec 4 at 20:33
Equivalents are fast but also dangerous to handle for limits.
– gimusi
Dec 4 at 20:33
Why? If one knows the rules… When I was a student, our professor used to say that L'Hospital's rule can be dangerous (there are some hypotheses no one ever thinks to check), and when it works, Taylor at order $1$ works as well.
– Bernard
Dec 4 at 20:39
Why? If one knows the rules… When I was a student, our professor used to say that L'Hospital's rule can be dangerous (there are some hypotheses no one ever thinks to check), and when it works, Taylor at order $1$ works as well.
– Bernard
Dec 4 at 20:39
1
1
I'm not referring of course to the use by an expert user like you but for not expert user it can be dangerous. Let consider for example the limit $frac{t-log(1+t)}{t^2}$ accorging to $log (1+t) sim t$ one could conclude that $$frac{t-log(1+t)}{t^2}sim frac{t-t}{t^2}=0$$ which is of course wrong.
– gimusi
Dec 4 at 20:44
I'm not referring of course to the use by an expert user like you but for not expert user it can be dangerous. Let consider for example the limit $frac{t-log(1+t)}{t^2}$ accorging to $log (1+t) sim t$ one could conclude that $$frac{t-log(1+t)}{t^2}sim frac{t-t}{t^2}=0$$ which is of course wrong.
– gimusi
Dec 4 at 20:44
1
1
I think you should always indicates that issue when you suggest to use equivalents to not expert users.
– gimusi
Dec 4 at 20:45
I think you should always indicates that issue when you suggest to use equivalents to not expert users.
– gimusi
Dec 4 at 20:45
2
2
I'm not claiming that the method is wrong, I'm claiming that we need to be aware about it to handle them and a not expert user can easily make a mistake. I would suggest for example use them in the form $log(1+t)=t+o(t)$ in order to take into account the rermainder term. But it is only a suggestion of course.
– gimusi
Dec 4 at 20:55
I'm not claiming that the method is wrong, I'm claiming that we need to be aware about it to handle them and a not expert user can easily make a mistake. I would suggest for example use them in the form $log(1+t)=t+o(t)$ in order to take into account the rermainder term. But it is only a suggestion of course.
– gimusi
Dec 4 at 20:55
|
show 1 more comment
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You don't necessarly need l'Hopital here and I suggest to avoid that when we can proceed by standard limits.
– gimusi
Dec 4 at 20:35